Gliese 581eextrasolar planet discovered in 2009 that has a mass 1.94 times that of Earth and is therefore 581extrasolar planetary system containing six planets. One of them, Gliese 581g, was the first planet to be found within the habitable zone of an extrasolar planetary system, the orbital region around a star in which an Earth-like planet could possess liquid water on its surface and possibly support life. Another, Gliese 581e, is the smallest planet seen in orbit around an ordinary main - sequence star other than the Sun.

Gliese

581e

581g was discovered in 2010 and has a mass at least 3.1 times that of Earth. Gliese 581g is the

fourth

sixth planet discovered around the red dwarf star Gliese 581, which has a mass 0.31 times that of the Sun

. Gliese 581

and is 20.4 light-years from Earth.

The Gliese 581 planetary system contains four planets, of which Gliese 581e is the smallest

Gliese 581g orbits its star every 36.56 days at a distance of 21.8 million km (13.6 million miles), which is within the habitable zone. Since it is so close to its star, the tidal forces on Gliese 581g have made its rotational period the same as its orbital period. Thus, on one side of the planet it is always day, and on the other side it is always night. However, the planet’s mass is such that, if it has an atmosphere, the atmosphere may be thick enough to avoid freezing on the night side. The best conditions for life would be found near the terminator, the dividing line between the day and night hemispheres, where surface temperatures would be slightly below freezing.

Gliese 581e, discovered in 2009, has a mass 1.94 times that of Earth and is the smallest planet in the Gliese 581 system. It is also the closest to its star, orbiting once every 3.15 days at a distance from Gliese 581 of 4.5 million km (2.8 million miles). Gliese 581e is a rocky planet like Earth, but it probably could not sustain life; it is too close to its star for liquid water to survive on its surface.

However, Gliese 581d, which orbits every 66.8 days and is 32.9 million km (20.5 million miles) from Gliese 581, is within the planetary system’s habitable zone.